I. Field of the Invention
This invention relates generally to automatic controls for boilers, and more particularly to a microprocessor-based sequencer capable of monitoring changes in load demand and for adjusting the firing rate of plural boilers in accordance with a base load algorithm.
II. Discussion of the Prior Art
Various systems are disclosed in the prior art for controlling the operation of multiple boilers in a coordinated fashion so that they jointly meet the load demands of a heating system or other industrial process. For example, the Bader U.S. Pat. No. 4,598,668 discloses a control system for operating any one of a plurality of parallel connected boilers as a lead boiler, with the others operating as lag boilers. The control mechanism described in the patent enables a lag boiler to assist the lead boiler during prolonged operating periods so that the load is jointly shared. This is said to extend equipment life and to improve the overall efficiency of the system. In the event of a sudden boiler failure, the failed boiler is effectively switched out of operation and then the load is adjusted between the remaining boilers.
The Bartels U.S. Pat. No. 4,513,910 describes a multi-boiler system and especially a control arrangement therefore for operating modulating motors and a fuel burner capable of operating in a low fire mode and a modulating mode. A pressure or temperature sensor provides signals proportional to the actual load being delivered which causes the boiler to be modulated to cause a boiler to operate in its low fire mode upon start-up but switching to a modulating mode when a high fire limit is reached.
The Shprecher et al. U.S. Pat. No. 5,042,431 describes a microprocessor-based sequencer for a multiple boiler heating system in which each boiler has an adjustable firing level of modulation at which it is turned on and an adjustable threshold level of modulation below which the next boiler is disabled from being turned on. Means are provided for continuously comparing the actual heating load to a system set-point whereby the total change in system output level required to produce a specified temperature within a given time period is developed. By appropriate firing and turnoff of plural boilers, the total workload can be shared. Both the Shprecher et al. patent and the Bader patent pertain to socalled parallel mode controls in which an operator selects the boilers to be run and which are brought up after a predetermined time delay to all follow the demand. The first boiler ignites and after a time delay, it follows the demand. At an adjustable point after the time delay has expired, the second boiler ignites and warms and then the first and second boiler both follow the demand. In the event of a falling demand, the boilers are shed in exactly the reverse order. While parallel operation allows multiple boilers to share a common load, it does not ensure that the system will be operating at optimum efficiency.
As those skilled in the art appreciate, the most efficient firing rate varies from boiler to boiler. A need, therefore, exists for a boiler sequencer or controller which will base load individual boilers at their most efficient firing rate. In a base load mode of operating three boilers, on original start-up, the first boiler carries the load until its firing rate reaches its programmable Add Boiler Load set-point which may, for example, be about 45 percent. At this time, the second boiler fires and is held at "low fire" for a time sufficient to alleviate damage due to thermal shock. It then follows the load in parallel with the first boiler until the second boiler reaches an Effect Base Load set point at, for example, 25 percent. The first boiler will increase in firing rate in increments along with the second boiler until the first boiler reaches its Preferred Load value. Any further changes in load demand will be satisfied by the second boiler swinging. Hence, the firing rate of the first boiler will not go higher than its Preferred Load set point until the second boiler has its firing rate equal or greater than the Advanced Base Load set point for that boiler. If the demand increases to the point where the second boiler reaches its Add Boiler set-point, the third boiler is brought on-line and will act as the modulating or swing boiler for the system. In this fashion, whenever possible, one or more boilers is allowed to operate at its preferred load at which its efficiency is a maximum. By providing an automated base load feature, considerable fuel savings over the parallel mode of boiler operation can be realized. Further, by allowing intermittent warm-up of the idle boilers, less repair and down-time are experienced.
As the firing rate requirements drop off, the lag boiler will have its firing rate lowered. The first boiler, if above its Preferred Load, will follow the lag boiler until the first boiler is at its preferred load. It then will not follow the lag boiler downward in load any further until the lag boiler reaches the Retard Base Load set-point. The first boiler will then again follow the lag boiler downward in load. The lag boiler will drop off-line when it reaches its Shed Boiler Load set-point.